1. Field of the Invention
This invention relates to a curable composition and more particularly to an epoxy resin-based curable composition which is cured by exposure to light and heat.
2. Description of the Prior Art
Various photocurable compositions have heretofore found extensive utility in various applications such as printing plates, coating materials, printing inks, etching resists, plating resists, solder resists, surface covering materials, varnishes, adhesive agents, and potting materials. Most of them are compositions which basically comprise a radically polymerizable polyfunctional (meth)acrylic ester and a radical photopolymerization initiator. This reaction system is characterized by exhibiting an excellent photocuring properties and offering great versatility in the combination of (meth)acrylic esters which can be used therefor. Since the curable compositions containing (meth)acrylic esters undergo marked voluminal shrinkage while being cured by the agency of light, however, the cured products thereof are generally deficient in substrate adhesiveness and in dimensional stability. In theory of reaction, the radical photopolymerization system has the problem that the cured product exhibits poor surface curing properties because the polymerization is inhibited by oxygen. Many polyfunctional (meth)acrylic esters give a strong effect on human skin and, in this respect, incur a problem of difficult handling.
Photocurable compositions produced from polyfunctional vinyl ethers or epoxy compounds by using such cationic photopolymerization initiators as diphenyl iodonium hexafluoroantimonate and triphenyl sulfonium tetrafluorophosphate have been also known. These photocurable compositions are characterized by undergoing small voluminal shrinkage during the course of curing, escaping the action of oxygen tending to inhibit polymerization, and having only a low stimulus to the human skin as compared with the acrylic ester-containing photocurable compositions, and, therefore constitute important chemically amplifying photoresists possessed of high sensitivity and high resolution. When such compounds as diphenyl iodonium hexafluoroantimonate and triphenyl sulfonium tetrafluorophosphate are used for the photocuring reaction of a polyfunctional vinyl ether or epoxy compound, a strongly acidic compound inevitably persists in the cured product because the cationic polymerization initiator, which is formed by exposure to light, is a strong acid. When these compositions are used as adhesive agents, solder resists, coating materials, and covering materials, therefore, the corrosion of a substrate by the acid poses a serious problem. Further, the cationic photopolymerization system, though not susceptible to the action of oxygen tending to inhibit reaction, has the reaction thereof seriously inhibited by water and the hydroxyl group of the reaction system. As a result, the photocuring system of this nature requires removal of impurities, selection of a functional group, and extremely high purity and, from the technical point of view, poses a problem of difficult handling.
Epoxy resin-based compositions have been known as thermosetting compositions which are capable of precluding corrosion of a substrate by a strongly acidic compound produced by exposure to light as described above and are not subject to the action of oxygen tending to inhibit a reaction.
Since the epoxy resins cured by the use of various amine type curing agents, for example, excel in mechanical strength, dimensional stability, adhesive strength, and resistance to chemicals, the epoxy resin-based curable compositions have been finding extensive utility in a wide variety of applications such as adhesive agents, potting materials, printed circuit boards, composite materials, coating materials, and printing inks. These curable compositions react relatively rapidly and this reaction proceeds while the compositions are being handled. Various technical studies, therefore, have been pursued and due improvements proposed with respect to the micro-capsulation of the curing agent and the control of the stubborn reaction. The epoxy resin-based compositions nevertheless exhibit many problems which have yet to be solved or improved an in terms of storage stability and reaction control.
Imidazole compounds are heterocyclic compounds containing a secondary amino group and a tertiary amino group. They are usable by themselves as a crosslinking agent for epoxy resins. Because of their excellent catalytic activity, however, they prove highly useful in most cases as a catalyst for the ring opening polymerization of an epoxy group and as a catalyst for the addition reaction of an epoxy group to carboxylic acids, phenols, alcohols, amines, or activated esters. Thus, they have been finding extensive utility in a wide range of industrial applications. When imidazoles are added to epoxy resins (epoxy compounds) or to the mixtures of epoxy resins with such crosslinking and curing agents as polyfunctional carboxylic acids, phenols, alcohols, and amines, however, they go to accelerate the reaction and consequently impair the storage stability.